Charged particle beam systems are used in a variety of applications, including the manufacturing, repair, and inspection of miniature devices, such as integrated circuits, magnetic recording heads, and photolithography masks. Dual beam systems often include a scanning electron microscope (SEM) that can provide a high-resolution image with minimal damage to the target, and an ion beam system, such as a focused or shaped beam system, that can be used to alter substrates and to form images.
One common application for a dual beam system is to expose a buried portion of a substrate and then to form an image of the exposed surface. For example, a focused or shaped ion beam can be used to make a vertical cut in a substrate to expose a cross sectional surface, and then an electron beam can be scanned over the newly exposed surface to form an image of it.
One difficulty with such systems is that the final lens of the scanning electron microscope produces a magnetic field, which alters the trajectory of the ion beam and also interferes with various other functions of the dual beam system. For example, an image or information about the composition of the substrate can be obtained by collecting secondary particles ejected as the primary ion beam strikes target. The magnetic field of the SEM, however, changes the path of the secondary particles and makes them difficult to collect.
When a work piece in a charged particle beam system is composed of an insulating material, such as quartz, the work piece tends to accumulate electrical charge that adversely affects the primary beams and secondary particles. One method of neutralizing the change entails the use of an electron flood gun that directs electrons to the work piece to neutralize positive charge. An electron flood gun differs from an electron microscope in that the flood gun lack precise optics options and produces a relatively broad beam of low energy electrons. The magnetic field of the SEM changes the path of the neutralizing electrons from the flood gun and makes it difficult to direct them accurately toward the work piece.
A common solution to this problem of the magnetic field interference is to turn off the SEM when using the ion beam or when using certain functions of the ion beam system. For example, the SEM can be switched off to allow collection of the ion beam induced secondary particles or when using a charge neutralization flood gun. Turning the SEM lens on and off creates its own set of problems.
The magnetic objective lens of an SEM uses a significant electrical current and therefore generates a significant amount of heat, the heat being proportional to the square of the current. The heat dissipated by an SEM causes components of the dual beam system to expand. The resolution of an SEM, being on the order of magnitude of nanometers requires a very stable physical platform, and the system therefore requires a significant amount of time after being turned on to reach thermal equilibrium and become stable. As the resolution of systems has increased, stability has become more important, and longer waits are required. Charged particle beam systems were originally used only in laboratories to analyze samples, and the time to reach thermal equilibrium was acceptable. Systems are now being used as production equipment and such delays are unacceptable.
U.S. Pat. No. 4,345,152 for a “Magnetic Lens” describes an electron lens that uses two coils having equal numbers of turns wound in opposite directions. By altering the allocation of current between the two coils, the magnetic field could be adjusted to focus the electron beam while maintaining a constant total current, and therefore a constant heat output. Using two lenses of equal turns allows the magnetic field to be varied or even cancelled without changing the total electrical current in the lens. Thus, the magnetic field could be eliminated without changing the heat output of the lens.
The surface viewed by the SEM is often oriented at a non-perpendicular angle to the SEM axis, so one part of the work piece is closer to the lens than another part. To compensate for the difference in distance, some SEMs change the focus of the objective lens during the scan and can therefore produce a clearer image. This is often referred to as “dynamic focusing.” Dynamic focusing requires the ability to rapidly change the magnetic field, which requires rapidly changing the electrical current in the objective lens coils. The coil inductance, which is related to the number of turns of the coil, resists a current change.
The two equal coils in U.S. Pat. No. 4,345,152 have high inductance and cannot be changed rapidly. It is also known to use a separate, small lens for dynamic focusing. Such lenses have low inductance, but changing the lens current changes the power dissipation of the lens, which can upset the thermal equilibrium of the system, thereby reducing resolution.
If one designed a dual beam system to compensate for the effects of a constant magnetic field from the SEM objective lens, the problem would not be solved completely, because the magnetic field is not constant. To keep the SEM in focus, the magnetic field of the objective lens is changed depending upon the height of the work piece, the magnification, and the electron energy. In some system, it is possible to reduce the operating variation in the magnetic field by using “retarding field optics,” that is, changing the voltage of the work piece to change the focus of the electron beam, rather than changing the magnetic field in the objective lens. In many dual beam systems, the FIB is mounted vertically and the SEM is mounted at an angle to view to vertical cross section cut by the FIB. A system in which the SEM is tilted cannot easily use retarding field options, since the tilt eliminates the symmetry of the retarding electric field and causes undesirable aberrations in the primary electron beam and difficulty in the collection of secondary electrons.